Accumulated Other Comprehensive Income/(Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	$ (995)	$ (897)	$ (364)
Other comprehensive income/(loss) before reclassifications	358	(82)	(536)
Amounts reclassified from accumulated other comprehensive (income)/loss	45	(16)	3
Other comprehensive income/(loss)	403	(98)	(533)
Balance at end of year	(592)	(995)	(897)
Foreign currency translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(994)	(897)	(359)
Other comprehensive income/(loss) before reclassifications	407	(97)	(538)
Amounts reclassified from accumulated other comprehensive (income)/loss	0	0	0
Other comprehensive income/(loss)	407	(97)	(538)
Balance at end of year	(587)	(994)	(897)
Derivative financial instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(1)	0	(5)
Other comprehensive income/(loss) before reclassifications	(49)	15	2
Amounts reclassified from accumulated other comprehensive (income)/loss	45	(16)	3
Other comprehensive income/(loss)	(4)	(1)	5
Balance at end of year	$ (5)	$ (1)	$ 0